Microwave pulse generators of this type are used in radar systems, in particular in pulsed radar systems, for exact distance measurements.
Such pulse generators should satisfy the following requirements:
a) The pulse duration of the microwave pulse has to lie in the range of one nanosecond, so that the pulsed radar can meet the requirement for high resolution. PA1 b) The pulsed radar method requires a high coherence of the microwave pulse carrier frequency and the pulse repetition rate. To meet this requirement, an oscillator has to start oscillating always with the same initial phase. PA1 c) The ratio of pulse period to pulse duration has to be large so that the following microwave pulse is transmitted only after all echoes from the target have been received. PA1 d) The energy in the microwave pulse generator must decay quickly after a pulse, i.e., the transition from the pulse duration to the pulse pause has to occur quickly in order to capture very small echoes. The dynamic range of a pulse radar is typically between approximately 80 to 100 dB. The microwave pulse has to have decayed by this amount within a very short time. The RC resonator which determines the frequency, therefore has to have a small Q-value.
Conventional CW microwave oscillators are not adapted to generate coherent pulse trains, because they typically have resonant circuits with a high Q-value (Q&gt;100) to maintain a sufficiently stable frequency. As a result, the settling time of such oscillators will be larger than 100 periods after being switched on, and then once again more than 100 periods until the oscillations have decayed after the oscillator is switched off. Consequently, special microwave pulse generators are very complex and produce a very broad spectrum from which the desired signal is filtered out with band pass filters. Disadvantageously, a microwave pulse generator of this type has a poor efficiency.
The German patent DE 44 01 350 C1 discloses a method for generating microwave pulses and a corresponding apparatus with a step recovery diode (SRD). The method and apparatus described therein satisfy the aforedescribed requirements (a) to (d), but have, as also discussed above, a very poor efficiency. The arrangement disclosed in the reference includes a pulse generator with a pulse differentiating circuit following the pulse generator. A subsequent matched network transmits the pulses to the step recovery diode which is integrated with a resonator. The resonator includes a capacitive and an inductive TEM line section together with the blocking capacitance of the step recovery diode. This arrangement already defines the center of the produced spectrum. Additional filtering with a band pass produces a microwave pulse with the carrier frequency f.sub.T and the pulse duration t.sub.p.
It is an object of the present invention to provide a microwave pulse generator which requires few components and produces a microwave pulse with a pulse duration t.sub.p in the range of one nanosecond. It is another object of the invention to improve the efficiency of the microwave pulse generator by at least a factor of 10 over conventional pulse generators.